Online compiler, debugger and visualizer
Online JavaScript compiler
Run JavaScript online on Node.js 22. Read input from the console, split code into modules, debug with breakpoints, test your functions against expected outputs, and watch arrays, objects, linked lists, trees and graphs change step by step in the visualizer.
- Free
- No sign-up
- Real compilers
- Works on phones
const readline = require("readline");const rl = readline.createInterface({ input: process.stdin }); rl.on("line", (line) => { const words = line.trim().split(/\s+/); console.log(words.reverse().join(" ")); rl.close();});What you can do
Real JavaScript (Node.js 22)
Your code is compiled and run by the real JavaScript toolchain on our servers, not a simulation, with the exact errors you would see on your own computer.
Input while it runs
Programs that read input (Scanner, input(), scanf, cin) ask for it in the console as they run, just like a terminal. Or prepare the input in advance.
Step-by-step debugger
Set breakpoints, step over, into and out, and watch variables and expressions change, right in the browser.
Visualizer
Replay your program line by line and see arrays, objects and the call stack drawn as it runs. Ideal for learning recursion and data structures.
Test cases
Add inputs with the expected output and run them all with one click. See which lines differ, like on a coding judge, for any code you write.
Errors explained
Compiler and runtime errors link to the exact line, with a plain-English explanation of what went wrong and how to fix it.
Help that sees your code
Ask why a test fails or what an error means. The assistant sees your files and the real output, and can fix the right line for you.
Code together live
Share a link and code with friends or students in real time: shared cursors, edits, runs and test results.
How it works
- 1
Click "Open the JavaScript compiler". A project with a ready-to-run main.js opens.
- 2
Write your code. Files save automatically in your browser as you type.
- 3
Press Run (Ctrl+Enter). If the program reads input, type it in the console, or press Debug (F5) to step through it.
- 4
Add test cases to check your program against expected outputs, or share a live link to code with someone.
Questions and answers
Is this browser JavaScript or Node.js?
Node.js 22, so require, modules and process.stdin work as they do on your computer.
Is it free?
Yes. Running, debugging, tests and live sessions are free, with no sign-up.
Where are my projects saved?
In your browser, on your device, automatically as you type. Nothing is lost when you close the tab. They stay until you delete them.
Is my code safe to run?
Every program runs in its own isolated sandbox with no internet access and strict limits on time, memory and processes, and is deleted when it finishes.